In the field of particle comminution, dry powder analytical sieving devices provide useful quality control functions. Such devices are commonly used to determine how much of a given comminuted sample taken from a production stream has been reduced to a predetermined size sufficiently fine to enable said fine particles to be sifted through a screen having holes of a predetermined size.
The time required to make such an analytic determination is generally of some significance, not only from the standpoint of the time spent by the person performing the analysis but, more importantly, because production of the product is usually continued while a production sample is being analyzed. Hence, analysis time is of appreciable practical significance, since the longer the analysis time the greater are the amounts of possible off-specification material being produced.
Presently available analytical sieving devices generally comprise a pair of cylindrical chambers stacked one atop the other on a common axis and separated by a sieving screen. A preweighed product sample is deposited onto the screen in the upper chamber and the chamber is then closed. Disposed within the lower chamber is a slotted rotor adapted to direct a jet curtain of air onto the underside of the screen. The upwardly directed air jet curtain emanates from a radially disposed slot on the upper rotor surface that extends generally from the center of the screen outwardly to its periphery. Upon rotation of such air jet rotor, the jet curtain effectively sweeps the entire undersurface of the screen during each 360.degree. revolution thereof. A vacuum vent in the lower chamber functions to permit withdrawal of the fine particles which pass through the screen under the action of an induced air flow supplemented by gravity and by the agitation of the powder sample by the upward passage of the jet curtain through the screen. The present invention comprises an improved construction for analytic sieving apparatus and for the air jet rotor assembly employed therein. In its broader aspects, the subject invention includes an improved jet rotor assembly configured to emit a selectively shaped air jet curtain of diametric extant across the entire sieve surface in association with a dependent skirt that conjointly functions to sweep the fines that have passed through the screen toward an exhaust channel. In its narrower aspects, such rotor assembly includes a diametric slot that is of linear character adjacent the axis of rotor rotation and which assumes a selectively oriented curved configuration as it approaches the circumference of the sieving screen. In general, when viewed in plan, the slot in the rotor from which the air jet curtain is emitted is of generally bowshaped configuration not unlike a portion of the letter "C".
Among the advantages attendant such improved jet rotor construction and slot configuration is an increase in the ratio of the area of the slot in proportion to the total area of the annulus that is swept by the curved portions of the slot in the vicinity of the screen periphery to provide for increased jet energy at such location and to thus reduce the tendency of particles to accumulate on the screen periphery. The new slot configuration also air washes the sample twice per rotor revolution and its unique shape causes the sample powder on the upper screen surface to be displaced upwardly and then inwardly toward the center of the screen as one curved slot passes beneath the powder and then upwardly and outwardly as the second slot advances therepast, thus effecting a radially oriented reciprocatory displacement of the powder on the screen surface. Such powder displacement appears to significantly increase both the rate of particle displacement and particle scatter thereby releasing more of the fine particles to be sieved for passage through the screen openings. All of the foregoing appear to result in reduced sieving times, for example, by as much as 15% on difficult to sieve powders at least when analyzing specimens on sieves with openings of 45 microns and finer.
In its preferred form, the improved sieving apparatus is adapted to be mounted atop and in fluid communication with a filter assembly designed to access the outward flow of fines after passage through the screen. A flat filter element within said filter assembly provides for collection of fines for weighing, further analysis or preservation or recycling of valuable materials.
A primary object of this invention is an improved analytic sieving device.
Another object of this invention is the provision of an improved sieving device characterized by improved efficiency and reduced analysis time.
More specific objects are to provide an improved rotor construction and air jet slot configuration for an analytic sieving device that functions to air wash the sample screen twice for every revolution of the rotor and which displaces the screen supported particulates in a reciprocatory pattern; and which selectively increases available air wash energy adjacent the screen circumferance to reduce particle accumulation thereat.
A further object of this invention is to provide an improved sieving device that reduces sieving time by up to 15% on "difficult to sieve" powders on sieves with openings of 45 microns and finer. Another object is to provide for convenient retrieval of fines in an analytic sieving device of compact size and modular layout. Other objects and advantages of the subject invention will become apparent from the following portions of this specification and from the appended drawings which illustrate, in accord with the mandate of the patent statutes, a presently preferred embodiment of an analytic sieving device that incorporates the principles of this invention.